Scattered X-ray radiation (sometimes referred to as secondary or off-axis radiation) is generally a serious problem in the field of radiography. Scattered X-ray radiation is a particularly serious problem in the field of X-ray patient diagnostic imaging procedures, such as mammographic procedures, where high contrast images are required to detect subtle changes in patient tissue.
Prior to the present invention, scattered X-ray radiation in patient diagnostic imaging procedures has been reduced through the use of a conventional linear focused scatter-reducing grid. The grid is interposed between the patient and an X-ray detector and tends to allow only the primary, information-containing radiation to pass to the detector while absorbing secondary or scattered radiation which contains no useful information about the patient tissue being irradiated to produce an X-ray image.
(05) Some conventional focused grids used in patient diagnostic imaging procedures generally comprise a plurality of X-ray opaque lead foil slats spaced apart and held in place by aluminum or fiber interspace filler. In focused grids, each of the lead foil slats, sometimes referred to as lamellae, are inclined relative to the plane of the film so as to be aimed edgewise towards the focal spot of the X-rays emanating from an X-ray source. Usually, during an imaging procedure, the standard practice is to move the focused grid in a lateral direction, perpendicular to the lamellae, so as to prevent the formation of a shadow pattern of grid lines on the X-ray image, which would appear if the grid were allowed to remain stationary. Such moving grids are known as Potter-Bucky grids.
One problem with conventional grids of the type described above is that the aluminum or fiber interspace filler material absorbs some of the primary, relatively low energy, information-containing X-ray radiation. Because some of the primary radiation is absorbed by the interspace material, the patient must be exposed to a higher dose of radiation than would be necessary if no grid were in place in order to compensate for the absorption losses imposed by the grid. It is an obvious goal in all radiography applications to expose the patient to the smallest amount of radiation needed to obtain an image having the highest image quality in terms of film blackening and contrast.
Another problem with such conventional focused grids of the parallel lamellae type described above is that they do not block scattered radiation components moving in a direction substantially parallel to the plane of the lamellae. Two-dimensional grids remove more scattered radiation for a given thickness of grid. However, the presence of walls at right angles mean that there is no direction which is perpendicular to all the walls, which makes moving the grids much more difficult.
U.S. Pat. No. 5,606,589 to Pellegrino, et al. discloses air cross grids for absorbing scattered secondary radiation and improving X-ray imaging in general radiography and in mammography. The grids are provided with a large plurality of open air passages extending through each grid panel. These passages are defined by two large pluralities of substantially parallel partition walls, respectively extending transverse to each other. Each grid panel is made by laminating a plurality of thin metal foil sheets photo-etched to create through openings defined by partition segments. The etched sheets are aligned and bonded to form the laminated grid panel, which is moved diagonally and a precise number of periods during the X-ray exposure to pass primary radiation through the air passages while absorbing scattered secondary radiation arriving along slanted paths. Proper movement of the grid is very critical, but is also difficult, resulting in significant reliability problems.
The method of Pellegrino, et al. produces sturdy cellular air cross grids having focused air passages offering radiation transmissivity about equal to the best linear grids presently available. However, it has been found that the etching method of Pellegrino, et al. does not produce grids with very fine and precise dimensions, as desired.
What is still desired are improved apparatuses and methods for making focused anti-scatter grids with more transmission and better uniformity. Preferably, such improved apparatuses and methods will be relatively easier, less time-consuming and less expensive than existing techniques for making focused anti-scatter grids.